A previously proposed current-detection coil is discussed in the British EPE Journal, vol. 3, No. 1, March 1993, pp. 51-59, where simple solid toroids or flexible cut-open coil bodies are proposed as annular body shapes. Furthermore, the use of circular cut-open plastic rings with a rectangular cross section, which can be bent for placement on a conductor and then return to their annular shape, is discussed therein. Each of the various annular bodies used in the current-detection coils discussed in the EPE Journal are used to form a Rogowski current transformer.
Although all of the current-detection coil embodiments of the EPE Journal can be made to exhibit good transfer characteristics, when they are mass-produced, however, virtually each unit will exhibit different transfer characteristics and therefore individual adjustments or electronic correction circuits must be used to achieve uniform results. The effect of small deviations of the geometry of Rogowski current transformer coils from the ideal shape on their transfer characteristics is known. One solution proposed is to use polished straight ceramic rods as winding supports. See German Journal Technische Mitteilungen AEG-Telefunken 60 (1970) 7, pp. 444-448. Because, however, a current-detection coil operating according to the Rogowski principle must surround a conductor on a closed path, the use of straight ceramic rods requires that a closed current-detection coil be made of four partial coils. In this case, the rectangular junction of the component parts causes considerable interference in the electromagnetic field. In order to remove this influence, the junction points are made field-free through the use of a shielding. The desired accuracy of the Rogowski current transformer is therefore associated with considerable expense for the manufacture of the current-detection coils.